Typically, telephones are all powered from the central office via a voltage supplied over a pair of telephone wires. The service is often referred to as POTS (plain old telephone service) quality lines. The CO (central office) normally supplies a -48 volts in an idle condition (called TALK BATTERY). When there is an incoming call, the CO superimposes an alternating ring signal upon the normal voltage to generate a signaling noise (phone ringing) at the remotely located telephone. The idle signal has specifications such that it is normally 48 volts and can, in some circumstances, be as high as 80 volts. The ringing signal adds additional potential such that the shock hazard under extreme conditions can be substantial and therefore the ring signal has to be interrupted.
It has always been desirable to have all telephones powered from high reliability CO, rather than locally so that telephones remain operational even though electrical power may have failed at the remotely located telephone site.
The introduction of digital signaling to telephones has allowed the capability of multiplexing more than one voice signal on a single pair of POTS quality lines. However, it is extremely desirable for any digital multiplexing approach to not require a change in the customer or CO equipment.
The present invention solves the problems of multiplexing more than one voice signal onto a single POTS line by attaching a COT (central office terminal) unit in the CO which interfaces several POTS lines and an RT (remote terminal) at a location remote from the COT but near a location with a plurality of customers (different telephone numbers). The RT receives power from the COT but detects ringing signals in the form of digital overhead signals on the line and directs a remotely generated ringing signal to a specific customer line. Each customer line may have a number of CPD (customer premise device(s)) such as telephones, faxes, modems, etc. In this application, reserve power is defined as the difference between the power the RT is consuming and the maximum available power. To minimize the amount of voltage applied by the COT, the RT also includes circuitry for detecting the reserve power available in a regulated power conversion unit located within the RT and returns a digital signal back to the COT when the reserve power in the RT falls below a prescribed level whereby the COT can increase the applied voltage to the lines interconnecting the COT and the RT thereby increasing the power available at the RT to supply any added loads or to provide power needed on high resistance POTS quality lines between COT and RT. The power conversion device in one embodiment of the inventive concept used a PWM (pulse-width modulator) which was designed to operate in the discontinuous conduction mode whereby maximum power transfer between the applied voltage to the pulse-width modulator and the output occurred at a 50% duty cycle. With this design, a point between 0% duty cycle, at which there would be no output from the PWM transformer and minimum current in the primary winding and the 50% duty cycle point can be picked as the reserve power call point. This is the point at which there is so little reserve power that a digital signal must be sent back to the COT to raise the input voltage and thus, change the duty cycle in a direction toward 0% which would give the required regulated output voltage called for by a PWM control circuit.
The concept described in this application and claimed by the claims herein is related to co-pending applications assigned to the same assignee as the present invention and filed on the same date. These applications are U.S. applications Ser. Nos. 07/655,677, 07/655 and 07/654,932. The application Ser. No. 07/654,932, while directed to the same general subject matter, comprises a different specification and all subject matter in said application is incorporated herein by reference.
It is thus an object of the present invention to provide an improved system for providing power from a COT to remotely located multi-line CPD's via the use of an RT containing a power supply and ring generator thereby allowing the use of digital signals either over the POTS lines or another source such as fiber optics to be received and control the signaling (ringing or forward disconnect) to the CPD's connected to the RT.